Normally, if target have 100 hp left with 125% counter attack and your sniper kill it with 800 damage, the target will deal 125 counter damage to your sniper not 1000.
This isn’t my video but I found the bug.
In video, at 9:58, Scarlett deal 292 damage to Caedmon with 125% counter attack and Aodhan’s fiend. Caedmon deal 365 counter damage to Scarlett (292 x 125% = 365). This will be possible if Caedmon have exactly 292 hp left and there is no bug.
And at 10:00, Eichelberg deal 379 damage to Cyprian who almost die with 125% counter attack and Aodhan’s fiend. Cyprian deal 473 counter damage to Eichelberg (379 x 125% = 473). I don’t think Cyprian have exactly 379 hp left. And he suppose to deal much less counter damage than that.